52100 Steel Billet Diesel Pistons ???? 7

[grandquads]

Hello, I am currently working on a project with a direct injected common rail diesel engine. I build many diesel racing engines, (drag, truck-tractor pull, etc...). With that being said I want to build a very high output street able engine. My problem is the pistons, cast pistons can't hold the extreme pressures and heat, and forged pistons wear the ring lands out in 15 to 20 thousand miles. Budget is also a major concern. I was wanting to machine a piston from an existing steel forging but none are readily available without a major order. Which leads me to a billet piece that I can machine then heat treat. I was thinking of 52100 steel for my billets, mainly due to its hardness (after treat) and its ability to hold (treat) under high short burst temperatures. I currently use this material for rocker arm and other high stress parts in my engines. My questions are about the thermal expansion of 52100 with various thicknesses (I.E. bore clearance)? Also its ability to run against a compacted graphite (iron) cylinder wall? I feel very good about the design, the pin bore, and ring lands holding tolerance for many cycles with no issues. I have attached a picture of what I'm wanting to build. The pic is a forged piston from a major diesel manufacturer. I am not worried about the weight difference between the cast aluminum and the steel piston. Thanks for any help and I'm open to all suggestions (good and bad).

 

[swall]

As you have already discovered, your biggest constraint will be obtaining suitable alloys in small quantities. I would suggest 410 stainless, which has decent strength to 1200F. CTE is 6.5. For reference purposes, 52100 has a CTE of 6.9.

 

[EdStainless]

Have you looked at a hot work tool steel?
I am thinking of H13, 19, 21 and so on.
They are not as hard, but they retain very good hot strength, resist thermal fatigue and are tough.
The Mo, W, and Co (depending on the grade) will all improve hot properties.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[grandquads]

(swall) The small quanities is not so much of a problem. Luckily for me a local machine shop/ material suplier has the drops of 52100 in a workable size. I have looked at the 410 and 17-4 stainless, the only issue there is I don't have a good source for small pieces. I do like the properties of the stainless. Just trying to weigh the cost vs strength. My cylinder pressures will reach 3000-3250 psi with nitrous and high boost levels. I feel the 52100 will be fine as well as the stainless, agian the stainless will require less clearence and that will help the rings and ring lands survive much longer.

(EdStainless) The wrist pins are possibly going to be made from H-21. Very good material (I hope) and finding 10 pcs at 4.25" dia and 4.5" length is another issue in my area. With that said I will check on the availibilty and the expansion rate.

Thank You guys for the Info,
additional comments welcome

 

[TVP]

What final hardness are you expecting to use for 52100? 52100 is usually hardened to > 60 HRC, and used in applications that need high resistance to contact fatigue, e.g. bearings or rollers. For a dynamically loaded part like the piston of an internal combustion engine piston, there will be multiple stress states: tension, compression, bending, and contact. Have you considered the very low fracture toughness of this material, especially for regions with high stress intensity (tension or bending)? I have a difficult time imagining an engine with very high cylinder pressure lasting very long with pistons made from a brittle material like 52100 hardened to ~ 60 HRC.

 

[redpicker]

I would have the same concerns as TVP. It would be quite possible that a piston from 52100 steel would not perform as well as a cast piston. Unfortunately, even tempering the material to a lower hardness does not help much, since the large carbides in the structure would still lead to lower fracture toughness.

To make a better choice of materials, you will need to define the required properties, then a manufacturing process that satisfies the economics of your situation. Off-hand, I'd say the properties you'd want to consider
1) hardness
2) fatigue strength
3) density
4) coefficient of thermal expansion (CTE)
5) toughness
6) temperature resistance
You don't need exact values for these properties, but at least some idea of what is needed. For example, the fatigue strength needs to be at least as good (probably better) than the cast materials you are trying to replace.

With this information, you can make an informed decision on material and, if the new material does not give you the performance you need, you at least have a good definition of what doesn't work.

rp

 

[Wrenchbender]

You say "forged pistons wear the ring lands out in 15 to 20 thousand miles", but you did not mention any other problems, or what the material is. Assuming your present pistons are steel, why not stay with the existing alloy and try carburizing it for better surface wear? You might also try carburizing a low carbon steel, e.g. S1, P_, or 4330? for a tough core to withstand the fatigue. Agree with the above posts on the low toughness of 52100, altho the high temperatures might work in your favor, but there are better choices, economics notwithstanding.

 

[grandquads]

Thanks for the reply's again. I am somewhat aware of the (brittleness) in 52100. We use them as rocker arms, valve spring retainers and sometimes push tubes and lifter rollers. With that said these parts do not see drastic changes in temperature over a short period of time. They are very high stressed as our ramp rates on the cam profile are very aggressive and the parts are very thin. Even though with the extreme Cylinder pressures we expect are higher stresses yet. As far as a hardness level I was thinking of staying with what we run for our valve train parts and that's 56-60. I'm not sold by any means on sticking with this material ( its easy to obtain). The forgings I spoke of earlier are aluminum forgings for racing applications. We have run these in previous builds and they tend to wear the ring lands prematurely in high cylinder pressure applications. We have even tried hard anodizing, and the exotic DLC coating with very little luck. The top ring is a Key Stone shaped ring (for self cleaning purposes) and they wear the ring land very quickly. The stock cast pistons have a hardened steel insert for the top "key stone" ring cast into them. Other production engine manufacturers use 4340 forgings in heavy duty applications. As stated before the piston I wish to make are not available in a steel forging nor is a factory forging From other manufacturer practical (too big) for my application. I am still considering the 410 stainless that (swall) mentioned before. Also I have also been looking at 4150 or a high carbon alloy that hardens fairly well (for ring lands and skirts). The ring package is going to be chrome-molly rings (exact material depends on piston material). The two have to live happily together. Obviously I'm not a metallurgist or an engineer by any means. I do have a ton of experience in engine building and we are always looking to be better, we are trying to make extreme engines daily, durable drivers. Thank you guys for your help, as I'm crossing into new territory for myself.

 

[grandquads]

Thank you guys again for the advice. I have teamed up with a local engineering group and they have suggested 8620 with a case hardening of around 50 or so, then finish grind to tolerence. They also suggest the 4340 ring package will be fine running against this material once its hardened (both rings and pistons). I am curious to your thoughts on this material.
Thanks Again

 

[metman]

I suggest using 8630 if availibility is suitable which will provide better core strength than 8620 to support the case from contact stresses transferred through the case while still maintaining high level of fracture toughness for fatigue of bending stress of the ring lands.

HRC 50 is a good choice for keeping brittleness from being an issue but I suggest specifying it as a minimum with allowable hardness as high as HRC 54 for optimizing wear and retention of ring land strength.

My copy of Steel Selection by Manfred Suess is not here at home. He provides formulae for calculating case depth dependeng on stress levels. If you are interested, let me know and I will dig out some info on this when I get the book from my workplace.

Another thing to consider is ring land root geometry. Are these typically designed with a radius between the land and the root of the groove? I would expect this to be the weak point because of high stress intensity at this junction. Is it possible to increase the land thickess and design the root groove with a full radius? Just some thoughts.

Design for RELIABILITY, manufacturability, and maintainability

 

[TVP]

grandquads,

I think that your local engineering group has provided some very reasonable alloys to use as a starting point. I agree that the part should be ground after case hardening. I also concur with metman that HRC 50 should be a minimum for the case hardness.

 

[grandquads]

Thank You guys for your inputs again. I have ordered material and should begin the roughing process next week. I will keep you posted on how this turns out. As far as the root of the lands, the base of the land(s) will measure .110". I plan to use a full radius on my grooving tool when they are rough machined. Also I will have a radius-ed stone when they are finished ground. Hopefully if the design comes out the way I plan there will not be any sharp corners (all radius or fillet). Also with the extreme pressures the pin boss area will see, there will only be .050 of free play between the connecting rod and the piston to eliminate any bending of pin. Also the pin will be 1.515" diameter H-11 tool steel pins with a tapered inner wall, (bought pieces for Cummins B series performance engines). Thank You again